In Latin a relative clause can be causal and the causal nature can be emphasized with quippe, ut, utpote or praesertim.
A causal relative clause can always be replaced with a causal clause, but not ...

One can insert the particle quippe in a relative clause to give it a causal or otherwise explaining tone.1
Does quippe require using conjunctive in the relative clause?
If not, are there some rules ...